1. Field of the Invention
The present invention relates to a camera system with an interchangeable objective lens and, more particularly, to a compensation of an error related to the fully-open aperture in the camera system.
2. Description of the Prior Art
In the field of a camera system with an interchangeable objective lens, an error related to the fully-open aperture occurs when the light measuring is carried out through the fully-open aperture under the TTL light measuring system.
Generally, in a TTL camera system with an interchangeable objective lens designed to measure light through the fully-open aperture, the light measuring output characteristics vary with respect to the details of the optical system, e.g., the location and diameter of the exit pupil, or to the details of the light measuring optical system, e.g., a deviation between the position where a light receiving element is located and the optical equivalent position of a film surface or a manner of diffusion of incident light to the light receiving element.
When a light amount of an object having a certain brightness is measured with a camera mounted with a certain interchangeable lens, the camera shows a different measured amount of light with respect to the type of mounted interchangeable lens. Therefore, unless the measured amount of light is compensated, the correct exposure control cannot be accomplished. Generally, the error caused by this measured amount of light (referred to as a fully-open aperture light measurement error) is expressed by a difference between the measured amount of light with a standard interchangeable lens (for example, a 50 mm/F1.4 lens) and that with another interchangeable lens.
Conventionally, various ways to compensate the fully-open aperture light measurement error have been proposed, and they can be divided into two groups. The first group is disclosed, for example, in Japanese Patent Laid-Open Publication No. 56-107221. According to this article, a means for generating a signal corresponding to the fully-open aperture light measurement error inherent to each type of interchangeable lens is provided in each interchangeable lens. The generated signal is transferred to the camera body for compensating the amount of light measured by the camera body. The generating means can be a type that generates a signal directly representing the fully-open aperture light measurement error or a type that generates a signal representing an amount of shift to be effected in a device for transmitting an aperture signal.
The second group is disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 55-143542 and 56-74226. According to these articles, the camera body calculates a fully-open aperture light measurement error based on information of fully-open aperture size or focal length, as obtained from an interchangeable lens. The calculated error is used for compensating a measured light amount.
According to the prior art camera systems, however, the amount of fully-open aperture light measurement error differs not only when a different type of lens is mounted on the same type of camera body, but also when the same type of lens is mounted on a different type of camera body having a different optical system for the light measurement. Therefore, the above-mentioned prior art camera system has follow disadvantages.
In regard to the first group, the generating means provided in each interchangeable lens is stored with a signal corresponding to the fully-open aperture light measurement error, but such a signal can be used properly in one particular type of camera body, and can not be used in the other types of camera bodies having a different light measuring system. In other words, when an interchangeable lens of the first group is mounted on a camera body of a type other than said one particular type, the signal produced from the generating means does not correspond to the fully-open aperture light measurement error and, therefore, the measured light amount will not be corrected properly. Although this disadvantage can be removed by providing the same optical light measurement system to each of different types of camera bodies, this is not appropriate because the design freedom and the specification of light measuring characteristic may be undesirably limited, resulting in a high manufacturing cost.
In regard to the second group, the camera body may be provided with a calculator which calculates various compensation values for different types of interchangeable lens to correct the error. But, such a calculator may be complicated and very difficult to prepare. Thus, according to the prior art, estimated corrections described below are carried out.
In the case of compensating the error of the measured amount of light based on the information of a fully-open aperture, the compensation corresponding to the diameter of the exit pupil of the interchangeable lens is carried out to some degree, but no compensation corresponding to the position of the exit pupil is carried out. And, in the case of compensating the error of the measured amount of light based on the information of focal length, the compensation corresponding to the position of the exit pupil is carried out to some degree, but no compensation corresponding to the diameter of the exit pupil is carried out. Thus, according to the prior art, compensation with a high accuracy can not be accomplished.
An error related to the fully-open aperture also occurs when exposure control is taking place. Generally, an image is formed by a picture-taking lens (objective lens) with a highest illumination at the center, where the optical axis passes, and the illumination is reduced towards the periphery of the image. Such a reduction of illumination at the peripheral portions is great particularly when the lens is set at the fully-open aperture position or around this position because of the phenomenon of the vignetting. Therefore, when the lens is set at the fully-open aperture position, the illumination of the image averagedly estimated within a predetermined extent of area, e.g., an 18 mm diameter circle, including its center will be considerably less than the case of a stopped-down aperture position due to the great reduction of illumination at the peripheral portions; although the central illumination may be identical regardless of the aperture position. Thus, in an exposure control system for controlling the exposure based on the calculated aperture value, the amount of light actually exposed will be averagedly estimated as an under exposure, when the aperture is to be set equal to or near the fully-open position according to the calculated aperture value. In such a way, an exposure error takes place.